Surface Acoustic Wave (SAW) devices are semiconductor devices that use surface acoustic waves whose energy is transmitted convergently on the surface of a solid. In general, a SAW device includes a layer of a piezoelectric material and one or more interdigitated transducer (IDT) electrodes formed on the piezoelectric layer. The surface acoustic wave may be excited by applying an electrical signal to an IDT electrode. Electrical signals are correspondingly generated across an opposite IDT electrode as surface acoustic waves pass the electrode. Typical piezoelectric materials include bulk monocrystals of quartz, as well as layers of LiNbO.sub.3, LiTaO.sub.3, AIN, or ZnO grown on a substrate. In a bulk acoustic wave (BAW) device, the energy of the acoustic waves may be transmitted through the solid, i.e., normal to the solid surface, or parallel to the surface. Both types of devices are often utilized for various signal processing applications. Surface acoustic wave devices are also used for sensor applications. The core components of these devices include the following: (a) piezoelectric layer to support acousto-electric transduction, (b) metal electrodes to trigger acousto-electric transduction in the piezoelectric layer, and (c) a medium for acoustic wave propagation.
In general, the active frequency (f) of a surface acoustic wave device is determined by the formula f=v/.lambda., where .lambda. is the wavelength and v is the propagation velocity of the surface acoustic wave in the piezoelectric material. The wavelength .lambda. is dependent on the spacing frequency of the interdigitated electrodes and the crystal orientation of the surface of the material through which the wave passes. Typical propagation velocities v for exemplary materials are as follows: 350 m/sec to 4000 m/sec for a monocrystalline LiNbO.sub.3 layer, and 3300 m/sec to 340 m/sec for a monocrystalline LiTaO.sub.3 layer. The propagation velocity v is relatively high at approximately 3000 m/sec for a ZnO film on a glass substrate.
The active frequency f can be increased either by increasing the propagation velocity v or by decreasing the wavelength .lambda.. Unfortunately, the propagation velocity is restricted by the material properties of the piezoelectric layer. The wavelength .lambda., which is determined by the width, spacing, and arrangement of the IDT electrodes, is limited by the lower limits of existing processing technologies. In a typical interdigitated electrode having an array of alternating equally spaced electrode fingers with a common width w and a common spacing s, for example, the wavelength is determined by the formula .lambda.=2s+2w. Other electrode arrangements will have other relationships between the wavelength, electrode width, and electrode spacing.
Submicron geometries may be difficult to fabricate using conventional materials, and long term reliability is typically limited by metal migration effects. For example, many existing optical lithography technologies cannot be used to fabricate a line/groove structure having a width of less than 0.8 microns. In addition, a narrower line width lowers the fabrication yield. For these reasons, the maximum frequency of many existing SAW devices in practical use is approximately 900 MHz. A surface acoustic wave device having interdigitated electrodes on a LiNbO.sub.3 substrate may have a surface acoustic wave velocity of 4003.6 m/s, a coupling coefficient of 5.57%, and a frequency temperature coefficient of -72 ppm/K, for example. In a device having alternating equally spaced interdigitated electrodes with 1 .mu.m wide electrodes and 1 .lambda.m spaces between electrodes, the frequency will be approximately 1.0 GHz. In order to achieve a 2.5 GHz device, the electrodes should have a width and spacing of approximately 0.4 .mu.m.
In the case of SAW devices including a piezoelectric film on a substrate, plural surface acoustic waves are excited if the sound velocity of the substrate is different than the surface acoustic wave velocity of the piezoelectric film. These surface acoustic waves are called zeroth mode waves, first mode waves, second mode waves, etc. according to the order of increasing velocity. The velocities of all modes depend on the substrate, as well as the piezoelectric film. The use of substrates having higher sound velocities results in higher velocities for all modes of the surface acoustic wave in the device. That is, the surface acoustic wave velocity increases in proportion to the sound velocity of the substrate.
Unilayer SAW devices are particularly limited in high frequency performance since state-of-the-art piezoelectric materials do not have sufficient SAW velocities to enable devices to be realized for GHz applications. Many materials that have sufficient SAW velocities are non-piezoelectric and hence cannot serve directly as acousto-electric transducers. The high SAW velocity of a non-piezoelectric material can be exploited by placing a piezoelectric layer on the surface of the non-piezoelectric (i.e. by utilizing multilayer SAW structures). By placing metal electrodes at either surface of the piezoelectric layer, a SAW can be launched in the multilayer structure. Such multilayer structures will allow acousto-electric transduction via the piezoelectric layer. The resultant wave will propagate in the multilayer structure and will be transduced at the receiver via reciprocity as in standard unilayer SAW devices. Thus, multilayer structures employ non-piezoelectric layers with high acoustic velocities to extend the high frequency operation limits imposed by existing lithographic constraints. A multilayer surface acoustic wave device is disclosed, for example, in a reference by Shiosaki et al. entitled High-Coupling and High-Velocity SAW Using ZnO and AIN Films on a Glass Substrate, and appearing in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-33, No. 3, May 1986. The SAW device disclosed by the Shiosaki et al. reference includes a borosilicate glass sheet substrate, a C-axis-oriented AIN film on the substrate, and a C-axis-oriented polycrystalline ZnO film on the AIN film opposite the substrate. Aluminum IDT electrodes are included between the AIN and the ZnO films. With this structure, a maximum coupling coefficient of 4.37% was reportedly obtained where the phase velocity was 5840 m/s. The frequency temperature coefficient of this device was 21.0 ppm/.degree. C. at 25.degree. C. The phase velocity of this device, however, is still relatively low. Accordingly, high frequency performance is limited.
A surface acoustic wave device having a relatively higher propagation velocity is disclosed in U.S. Pat. No. 5,221,870 to Nakahata et al. The patent discloses a SAW device having a silicon semiconductor substrate, a diamond film on the substrate, a ZnO piezoelectric layer on the diamond layer, and interdigitated transducer electrodes on the piezoelectric layer. For the diamond film, both a single crystal and polycrystalline diamond film are suitable. However, a monocrystalline film is more favorable, because there is less acoustic scattering in monocrystalline diamond as compared to polycrystalline diamond.
Diamond is a preferred material for many semiconductor devices because of its hardness, relatively large bandgap, high temperature performance, high thermal conductivity, and radiation resistance. Moreover, diamond is desirable for SAW devices because it has relatively large values of acoustic velocities. See, for example, "SAW Propagation Characteristics and Fabrication Technology of Piezoelectric Thin Film/Diamond Structure", by Yamanouchi et al., 1989 Ultrasonics Symposium, pp. 351-354, 1989. Combining diamond with relatively low velocity piezoelectric materials also results in higher SAW velocities; thus, the demands on line spacing may be reduced for a given frequency of operation as disclosed, for example, in "High Frequency Bandpass Filter Using Polycrystalline Diamond", by Shikata et al., Diamond and Diamond Related Materials, 2(1993), pp. 1197-1202.
Moreover, although the surface orientation of diamond will affect the absolute values of acoustic wave propagation, on average, longitudinal waves propagate at 18,000 m/s, shear waves at 12,000 m/s, and surface acoustic waves at 11,000 m/s. The SAW velocity in diamond is about three times higher than in LiNbO.sub.3. In particular, a 2.4 Gigahertz (GHz) diamond device using a .lambda..sub.o /4 design would require a one micron line and spacing geometry. Thus larger feature sizes can be used with diamond to fabricate devices that operate at frequencies in excess of 2.5 GHz. In contrast, 0.25-0.5 micron feature sizes would be required in devices fabricated from conventional SAW materials. Since the feature size of diamond-based devices can be larger than in other materials, current IDT feature sizes can be used, and advanced lithography tools are typically not required for device fabrication. Metal migration effects are also minimized in high frequency SAW devices. The ability to fabricate devices that can operate at frequencies in excess of 1.0 GHz without requiring submicron technology greatly increases yield and quality and decreases concomitant manufacturing costs.
The high acoustic velocity of diamond based devices is also advantageous for bulk acoustic wave BAW devices. As an example, in the resonator structure, as the frequency of operation increases the layer thickness decreases. For conventional materials, the thicknesses are too thin to be manufactured since they are not mechanically robust enough to be handled. For diamond BAW's operating at high frequency operation, the high velocity of diamond allows for thicker devices that have additional mechanical robustness than conventional materials.
One specific structure proposed for realizing high frequency SAW devices using diamond includes the use of ZnO. This is more fully described in U.S. Pat. No. 5,160,869 Nakahata et al. Proof-of-concept devices have also been demonstrated for operation up to 4.7 GHz. However, in order for the ZnO/diamond structure to be commercially viable, device improvements are still required. Many of the current device performance issues are related to the material properties of the ZnO/diamond structure. These are related to the structural, mechanical and electrical properties of ZnO/diamond films.
The commercial viability will typically be determined by whether the electromechanical coupling and propagation loss are improved to the levels required for commercial device performance. Electromechanical coupling is strongly influenced by the c-axis orientation of the ZnO, the resistivity of the ZnO and the polarity of the ZnO grains. The propagation loss is influenced by the c-axis orientation, the surface roughness, the resistivity, the grain size and the grain boundary integrity.
The development of high quality ZnO/diamond for high frequency applications has unique challenges that were not required for the development of ZnO materials at lower frequencies. The first challenge is that the material properties are strongly influenced by the substrate for ZnO deposition. Diamond is desirable due to its high sound velocity. As with any material selection, there are also substrate issues that pose unique obstacles. The surface energy of diamond is extremely high. This makes adhesion of materials to this surface difficult. It also strongly influences the nucleation of ZnO on diamond, which may affect the orientation of the ZnO grains. Additionally, oxygen, which is present in the deposition of ZnO, promotes the formation of graphite when diamond is exposed to energetic environments. A second challenge that was not as stringent for low frequency applications is the propagation loss (.alpha.) which has a frequency dependence. The propagation loss follows the general expression of .alpha.=Df.sup.2 where D is a constant and f is the frequency.
With respect to resistivity, an important difference occurs for ZnO/diamond relative to the prior art. For the devices in the prior art, the resistivity, and hence piezoelectric performance, is typically sufficient for high frequency operation (&gt;100 MHz). It was in the area of low frequency operation [see, e.g., U.S. Pat. No. 4,164,676] that the resistivity and piezoelectric performance needed to be improved for commercial applications. For the structure of ZnO/diamond, the device performance is not sufficient for device applications and additional improvements are required for high frequency operation.
The use of dopants to ZnO have also been used to enhance epitaxy. Dopants of Ni, Fe and Cu were selected for epitaxial deposition of ZnO on sapphire. (See, for example U.S. Pat. Nos. 5,432,397 and 5,532,537). In this device configuration, it may be desirable to orient the c-axis parallel to the sapphire substrate (the c-axis was perpendicular to the substrate in prior art structures). The use of compositional changes has also been investigated for epitaxial ZnO/diamond. For example, Li doping has been investigated to produce highly resistive epitaxial films on (111) diamond. Carbide formers on diamond, such as Ta, Mo, Zr, Y, Sc, Nb, Hf, W and Si, may also be suitable dopants. Additionally, isoelectronic traps such as Ge, C, Sn and Pb may also be valuable.
As described above, ZnO/diamond is a promising material for high frequency SAW and BAW applications. Composition modifications may also provide effective strategies for improving the properties of ZnO. Unfortunately, the deposition of ZnO is strongly influenced by the substrate. Additionally, the material requirements are more stringent for higher frequency operation. It is also difficult to know a priori what result compositional specific changes have on the performance of ZnO. Therefore, it is desirable to investigate and identify compositional dopants that are suitable for the optimization of ZnO/diamond to produce devices with sufficiently low losses so that they are suitable for commercial applications.